Essentialism
Essentialism, within feminism and anti-racist critiques, is the view that there are inherent, likely biological, differences between people's mental, emotional, and physical abilities, based on grosser biological differences such as sex, race, or other factors.
In other words, gender essentialism posits that there are essential characteristics of male and female in addition to the obvious, observable, physical dimorphisms. In racist thought, the notion would be that the "races" (a notion that is itself fraught with definitional difficulty) are comprised not just of various morphological differences (skin color, hair texture, eye shape, and so on) but of intellectual, emotional, and talent differences. Intellect is the characteristic most often discussed in these terms, but "sense of rhythm," "inability to feel compassion," and "unfriendliness" are among the many other traits which often show up in assumptions of racial essence.
Essentialism thus defines the relevant factors in part on observable phenomena, and in part on stereotyped differences, and presumes a causal correlation between the observed differences and the stereotypes. For instance, an essentialist take on the idea that men and women have different intelligence and mental abilities would start from the fact that various intelligence tests suggest that (most) women are less capable of certain types of abstract thought than (most) men (or, phrased more neutrally, that the sexes have different abilities). Since this can be measured quantifiably (on intelligence tests) and demonstrated in social outcomes (there are fewer female mathematicians than male mathematicians), the essentialist then concludes that there is an inherent, essential, likely biological cause for the differences.
History and modern trends
While essentialism has fallen out of favor for distinguishing among, for instance, the Irish and the English; or the English working class and the English nobility; it continues to have adherents regarding ethnicity and gender. A major trend of late 20th-century and early 21st century biology, sociobiology and related evolutionary psychology, have effectively updated essentialist arguments in the age of statistics and genomics. After being long discredited in scientific circles, racial essentialism (aka "racism") saw a surge of popularity following the publication of The Bell Curve, which employed a variety of statistical hand-waving techniques to examine socio-economic facts of unequal distribution of wealth, access to resources, and power, and concluded that inherent differences among the "races," as opposed to discrimination and social bias, was to blame. Stephen Jay Gould, a foremost critic of essentialist thinking in the history of the science of "race," updated his major work in the history of such discrimination, The Mismeasure of Man, to account for these new trends, and a number of other scientists took this on as well.
Gender essentialism has undergone a significant resurgence in science due primarily to work done in the sociobiology/evolutionary psychology field, and to certain largely anecdotal studies of high-profile sex reassignment cases; see esp. John Money. Scholars Carol Tavris and Anne Fausto-Sterling have looked at gender as Gould did at race, with their respective works, Mismeasure of Woman: Why Women Are Not the Better Sex, the Inferior Sex, or the Opposite Sex and Myths of Gender. Critics such as Gould and Fausto-Sterling take the position that the essentialists are simply replicating their own biases in their research, as has happened historically time and again when scientists attempt to inquire into the subject of popular prejudices. The recent popularity of gender essentialism hit the mainstream press when Harvard president Larry Summers attributed the lesser number of women scientists not to discrimination or unconscious bias but to their inherent abilities, interests, and life choices.
Sexuality and sexual behavior has been a particular site of scientific inquiry aimed at discovering essential (read: biological (read: genetic)) differences. Since the late 1800s and the work of Havelock Ellis, Freud, Magnum Hirschfield, and others; through the sexology research of Kinsey; to modern "gay science" like Simon LeVay, gay-positive scientists, sociologists, and psychologists have made essentialist arguments, and argued for scientific examinations of sexuality, usually explicitly campaigning for elimination of harsh criminal and social sanctions. Contemporary gay-positive scientists seek to demonstrate the essential character of sexual attraction in order to justify legal protections for same-sex relationships and queer-identified people against social prejudice. Research on sexuality is also predictably pushed by an anti-gay agenda, which would like to be able to "fix" homosexuality. (The latter ideas have been oft-explored in SF: See, e.g., Sheri S. Tepper's The Gate to Women's Country, which blithely posits that homosexuality had been "fixed"; and -- (need some of the other various cites which have critiqued the idea or expressed concern about the possible political fall-out).)
When trying to be politically correct, some sociobiologists and related theorists argue, as do difference feminists, that difference is not a justification for negative discrimination, but only for positive discrimination. See, e.g., Jared Diamond (of Guns, Germs, and Steel fame) and Steven Pinker (The Language Instinct, The Blank Slate). However, not all such adherents take a positive pro-social equality stance. Many express a variety of out-and-out racism and sexism. The scientific basis for these positions have been challenged by (among others)
The fields that support essentialism have also been challenged on pragmatic grounds by social justice advocates, who argue that (a) this is an unreliable field of inquiry because it is so susceptible to unconscious bias of the research; (b) it is only "sexy" to study because of the social biases that exist, and is therefore a misuse of research funds (a critique that rather accepts the social constructivist critique of essentialism); (c) it is dangerous because whatever differences there are ought not be discovered because they will be misused (a critique that is politically naive although certainly accurate in its prediction); (d) it is a waste of research energy and funds because, at present, our knowledge is such that we cannot even design experiments to correct for the wide variety of social effects; and so on.